Steam turbine airfoils or buckets are positioned around a turbine wheel at regular intervals. The buckets are assembled by inserting them one at a time axially into an opening on the wheel surface and then sliding the buckets circumferentially about the wheel. The buckets are attached by complimentary male and female dovetails. In order to close the assembly, however, the last blade must be restrained by means other than a dovetail. This last bucket, the closure bucket, generally is pinned to the wheel or affixed to its adjacent neighbors by keys.
The curved nature of the round skirt buckets, however, does not lend themselves to the use of keys in the solid portion of the skirt, i.e., the dovetail, so as to restrain the closure bucket from centrifugal forces. Axially positioned keys also are undesirable because the engagement with the adjacent buckets is very small so as to limit shear strength. Further, keys placed at an angle to the machine axis may be undesirable because the curved nature of the round skirt creates an irregular shear plane that does not intersect the center of the key at its thickest part. This may force the use of a closure block fixed to the wheel using pins. A closure block generally is a bucket without an airfoil. By removing the airfoil, the mass and centrifugal load is reduced, thereby lowering stress on the wheel pins to an acceptable level. Because the closure block has no airfoil, however, there is an opening in the steam path with detrimental effects on performance.
There is a desire therefore for an improved closure bucket that can support an airfoil without undue stress. Such a bucket should increase overall system performance.